You might think you can get by on 5 or 6 hours' sleep a night, but people who get less than 7 hours are more likely to have mood or mental health problems.

A severe lack of sleep has been linked to mood disorders, depression, anxiety and Alzheimer's disease. But much less is known about the effects of skimping on a little sleep each night, missing the recommended amount by an hour or so.

To find out the effect of low-level sleep loss on mental health, Kelly Sullivan and Collins Ordiah at Georgia Southern University analysed data from a telephone survey of over 20,000 people in the US. Respondents were asked about their sleep habits as well as their mood over the past 30 days.

Feeling hopeless

Around a quarter of participants said they got 6 hours of sleep or less, and these people were around 70 per cent more likely to report signs of mental health problems compared to those who got the recommended amount of sleep.

Compared with people who slept between 7 and 9 hours a night, people who got less than 5 hours were three to four times more likely to say they experienced depression, nervousness, restlessness or feeling hopeless in the last month.

These findings make sense given what we know about the connection between sleep and mental health, Sullivan says, but it is impossible to tell from this kind of study whether a lack of sleep causes mental health problems, or the other way round. "Symptoms for anxiety, depression and insomnia overlap, and we're aware that worsening of psychological symptoms can contribute to sleep challenges and vice-versa."

Daylight savings

However, the results are important given how many of us make do with not quite enough sleep, Sullivan says. In the UK, 40 per cent of adults report getting less than 6 hours a night.

"Many previous studies have focused on extreme sleep deficiency, but chronic mild sleep restriction is more common," says Sullivan. "These data likely offer a more accurate depiction of real life conditions," she says.

Steven Lockley, at Harvard Medical School, isn't surprised that getting just one hour less than the recommended amount of sleep may have an effect. "The hour we lose in the spring when going to daylight savings causes a 17 per cent increase in car crashes on the Monday morning and a 5 per cent increase in heart attacks in the 3 weeks after," he says.